1. Field of the Invention
The present invention relates to a mobile satellite communication system, and more particularly, to an antenna apparatus provided at a mobile station for communication with a satellite.
2. Description of the Related Art
In a mobile satellite communication system such as MSAT, where communication is carried out between a mobile station and a satellite, a prior art antenna apparatus provided at the mobile station is comprised of a single antenna element, to thereby obtain a compact configuration. Such a single antenna element is a helical-type mast antenna whose directivity can be adjusted by changing the length or pitch of a helical conductor element. Therefore, when the mobile station moves thus changing its location, a beam direction, i.e., a directivity of the antenna element always must face the satellite.
On the other hand, generally, in a mobile satellite communication system, a transmitting frequency (TX) of a signal transmitted from the mobile station is different from a receiving frequency (RX) of a signal received by the mobile station, to thereby avoid radio interference. For example, the transmitting frequency TX is 1.660 GHz, and the receiving frequency RX is 1.559 GHz.
In the above-mentioned prior art antenna apparatus, however, the directivity of the antenna element which is, in this case, the elevation angle of the antenna element, is dependent upon the frequency thereof. Therefore, when the directivity of the antenna element in relation to the satellite is optimum with respect to the receiving frequency, the directivity of the antenna element in relation to the satellite is not optimum with respect to the transmitting frequency. Conversely, when the directivity of the antenna element in relation to the satellite is optimum with respect to the transmitting frequency, the directivity of the antenna element to the satellite is not optimum with respect to the receiving frequency. This will be explained later in detail.